The afterglow and kilonova of the short GRB 160821B

E. Troja, A. J. Castro-Tirado, J. Becerra González, Y. Hu, G. S. Ryan, S. B. Cenko, R. Ricci, G. Novara, R. Sánchez-Rámirez, J. A. Acosta-Pulido, K. D. Ackley, M. D. Caballero García, S. S. Eikenberry, S. Guziy, S. Jeong, A. Y. Lien, I. Márquez, S. B. Pandey, I. H. Park, T. SakamotoJ. C. Tello, I. V. Sokolov, V. V. Sokolov, A. Tiengo, A. F. Valeev, B. B. Zhang, S. Veilleux

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GRB 160821B is a short duration gamma-ray burst (GRB) detected and localized by the Neil Gehrels Swift Observatory in the outskirts of a spiral galaxy at z = 0.1613, at a projected physical offset of 16 kpc from the galaxy's center. We present X-ray, optical/nIR, and radio observations of its counterpart and model them with two distinct components of emission: A standard afterglow, arising from the interaction of the relativistic jet with the surrounding medium, and a kilonova, powered by the radioactive decay of the sub-relativistic ejecta. Broadband modelling of the afterglowdata reveals a weak reverse shock propagating backward into the jet, and a likely jet-break at 3.5 d. This is consistent with a structured jet seen slightly off-axis (θview ∼ θcore) while expanding into a low-density medium (n ≈ 10-3 cm-3). Analysis of the kilonova properties suggests a rapid evolution towards red colours, similar toAT2017gfo, and a low-nIR luminosity, possibly due to the presence of a long-lived neutron star. The global properties of the environment, the inferred low mass (Mej ≲ 0.006 Mθ) and velocities (vej ≳ 0.05c) of lanthanide-rich ejecta are consistent with a binary neutron star merger progenitor. 

Original languageEnglish
Pages (from-to)2104-2116
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Publication statusPublished - Oct 2019


  • Gammaray burst: General
  • Gravitational waves
  • Nuclear reactions, nucleosynthesis, abundances
  • Stars: neutron

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